The use of multi-level modulation formats is one of the most effective ways of enhancing spectral efficiency in optical communication systems and provides a cost-effective method of increasing the transmission capacity. Many modulation formats are known for generating multilevel optical signals, including quadrature amplitude modulation (QAM), where both of the in-phase and quadrature components are modulated in a multi-level manner. Differential modulation formats, such as differential quaternary phase-shift-keying (DQPSK) that allows the transmission of 2 bits/symbol, are also appealing as they provide higher robustness against fiber nonlinearities. Polarization multiplexing can also be used to increase the total capacity of an optical communications system link.
Depending on the modulation format which is used, different techniques are required to receive the transmitted bits. Currently, a transmitted optical signal needs to be converted into the electrical domain in order to extract and process the transmitted communications traffic. Where the modulation format of the transmitted optical signal includes a phase modulation the most common method of converting the optical signal into the electrical domain using digital coherent optical receivers, such as reported by Y. Mori et al, “Unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals using digital coherent optical receiver”, OECC/ACOFT 2008, Sidney 7-10 Jul. 2008. While there are commercially available solutions able to process signals in the electrical domain at up to 50 GHz, electronic approaches become increasingly complex and expensive as the frequency gets higher.